#### list comprehension

```Hi all,

I wonder what would be the Perl notation for 'set-builders', as exposed
in this wikipedia article:

https://en.wikipedia.org/wiki/Set-builder_notation#Parallels_in_programming_languages

This is the Python notation:

Example 1: {l for l in L}
Example 2: {(k, x) for k in K for x in X if P(x)}

This is another example in Python:

s = {v for v in 'ABCDABCD' if v not in 'CB'}

https://en.wikipedia.org/wiki/List_comprehension#Similar_constructs

I have been playing with the code below. Nevertheless, I am unsure on
how to use the code to define a set.

Cheers!

<--- Code
#!/usr/bin/env perl6

my @L = 1 .. 10;
my @K = 1 .. 10;
my @X = 5 .. 15;

say "Example 1:";
for @L -> \$l {
print "\$l " if \$l ∈ @L;
}

say "\nExample 2:";
for @K -> \$k { for @X -> \$x {
print "(\$k, \$x), " if (\$k ∈ @K and \$x ∈ @X and \$x < 8);
}}
<---

--
(≧∇≦) Mimosinnet (Linux User: #463211)
```
 0
mimosinnet
2/10/2019 4:26:30 PM
perl.perl6.users 1202 articles. 0 followers.

6 Replies
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Similar Articles

[PageSpeed] 20

```In

{l for l in L}

The reason it is in `{}` is to create a Set from iterating over `L`.

> In Python, the set-builder's braces are replaced with square brackets, pa=
rentheses, or curly braces, giving list, generator, and set objects, respec=
tively.

So in Python:

[ l for l in L ]     gives a list
( l for l in L )     gives a generator
{ l for l in L }     gives a set

In Perl6 those would most likely be written as:

L.List   or   L.Array   or   L.list
L.Seq
L.Set

---

The way to do that is

my \L =3D ((1..10) xx 3).flat.pick(*).list;

set( L ) # A
L.Set # B

my %set is SetHash;
{ ++%set{\$_} for L }  # C

# D
do {
# add the {} syntax to create a Set (lexically)
my sub circumfix:=C2=AB{ }=C2=BB ( \L ) { L.Set };

{ \$_ for L } # <--
}

Something that seems similar to me is `unique`

.say for L.unique;

By that I mean, some places where you would use a Set, it makes sense

---

As for `{(k, x) for k in K for x in X if P(x)}`

The easiest one to directly translate appears to be the Scala one

my \K =3D 1..10;
my \X =3D 5..15;

# for (k <- K; x <- X if P(x)) yield (k,x)
Set.new: gather {
for K -> \k {
for X -> \x {
if P(x) {
take (k,x);
}
}
}
}

Other ways:

Set.new: (K X[,] X).grep: -> ( \k, \x ) { P(x) }

Set.new: K X[,] X.grep: &P

Set.new: K X[,] X.grep: &P

Set.new: ( -> ( \k, \x ) { (k,x) if P(x) } for K X[,] X )

Set.new: ( -> \x { |(-> \k { (k,x) if P x } for K) } for X)

On Sun, Feb 10, 2019 at 10:26 AM mimosinnet <mimosinnet@gmail.com> wrote:
>
> Hi all,
>
> I wonder what would be the Perl notation for 'set-builders', as exposed
> in this wikipedia article:
>
> https://en.wikipedia.org/wiki/Set-builder_notation#Parallels_in_programmi=
ng_languages
>
> This is the Python notation:
>
> Example 1: {l for l in L}
> Example 2: {(k, x) for k in K for x in X if P(x)}
>
> This is another example in Python:
>
> s =3D {v for v in 'ABCDABCD' if v not in 'CB'}
>
> https://en.wikipedia.org/wiki/List_comprehension#Similar_constructs
>
> I have been playing with the code below. Nevertheless, I am unsure on
> how to use the code to define a set.
>
> Cheers!
>
> <--- Code
> #!/usr/bin/env perl6
>
> my @L =3D 1 .. 10;
> my @K =3D 1 .. 10;
> my @X =3D 5 .. 15;
>
> say "Example 1:";
> for @L -> \$l {
>   print "\$l " if \$l =E2=88=88 @L;
> }
>
> say "\nExample 2:";
> for @K -> \$k { for @X -> \$x {
>     print "(\$k, \$x), " if (\$k =E2=88=88 @K and \$x =E2=88=88 @X and \$x < 8=
);
> }}
> <---
>
> --
> (=E2=89=A7=E2=88=87=E2=89=A6) Mimosinnet (Linux User: #463211)
```
 0
b2gills
2/10/2019 6:05:35 PM
```Dear Brad,

Thanks very much for the answer. I have been playing with your examples in the
code below (and learned a lot!). Based on your insight, I would suggest these
solutions to be added to the wikipedia:
https://en.wikipedia.org/wiki/Set-builder_notation#Parallels_in_programming_languages

Example1: Set.new: gather { for L { take \$_ } };
Example2: Set.new: cross( K, X.grep: P(x) );

Cheers!

<--- Working code
my \L = 1..10; my \K = 1..10; my \X = 5..15;

# Example 1
my \$e1 = Set.new: gather { for L { take \$_ } };

# Example 2
my \$s1 = Set.new: gather {
for K -> \k {
for X -> \x {
if x < 8 {
take (k,x);
}
}
}
}
my \$s2 = Set.new: (K X[,] X).grep: -> ( \k, \x ) { x < 8 };
my \$s3 = Set.new: ( -> \x { |(-> \k { (k,x) if x < 8 } for K) } for X );
my \$s4 = Set.new: gather { -> \k { -> \x { take (k,x) if x < 8; } for X } for K }
my \$s5 = Set.new: cross( K, X.grep: * < 8 );

say \$e1; say \$s1; say \$s2; say \$s3; say \$s4; say \$s5;
<---

El Sunday, 10 de February del 2019 a les 12:05, Brad Gilbert va
escriure:

>In
>
>     {l for l in L}
>
>The reason it is in `{}` is to create a Set from iterating over `L`.
>
>> In Python, the set-builder's braces are replaced with square brackets, parentheses, or curly braces, giving list, generator, and set objects, respectively.
>
>So in Python:
>
>    [ l for l in L ]     gives a list
>    ( l for l in L )     gives a generator
>    { l for l in L }     gives a set
>
>In Perl6 those would most likely be written as:
>
>    L.List   or   L.Array   or   L.list
>    L.Seq
>    L.Set
>
>---
>
>The way to do that is
>
>    my \L = ((1..10) xx 3).flat.pick(*).list;
>
>    set( L ) # A
>    L.Set # B
>
>    my %set is SetHash;
>    { ++%set{\$_} for L }  # C
>
>    # D
>    do {
>        # add the {} syntax to create a Set (lexically)
>        my sub circumfix:«{ }» ( \L ) { L.Set };
>
>        { \$_ for L } # <--
>    }
>
>Something that seems similar to me is `unique`
>
>    .say for L.unique;
>
>By that I mean, some places where you would use a Set, it makes sense
>
>---
>
>As for `{(k, x) for k in K for x in X if P(x)}`
>
>The easiest one to directly translate appears to be the Scala one
>
>    my \K = 1..10;
>    my \X = 5..15;
>
>    # for (k <- K; x <- X if P(x)) yield (k,x)
>    Set.new: gather {
>        for K -> \k {
>            for X -> \x {
>                if P(x) {
>                    take (k,x);
>                }
>            }
>        }
>    }
>
>Other ways:
>
>    Set.new: (K X[,] X).grep: -> ( \k, \x ) { P(x) }
>
>    Set.new: K X[,] X.grep: &P
>
>    Set.new: K X[,] X.grep: &P
>
>    Set.new: ( -> ( \k, \x ) { (k,x) if P(x) } for K X[,] X )
>
>    Set.new: ( -> \x { |(-> \k { (k,x) if P x } for K) } for X)
>
>On Sun, Feb 10, 2019 at 10:26 AM mimosinnet <mimosinnet@gmail.com> wrote:
>>
>> Hi all,
>>
>> I wonder what would be the Perl notation for 'set-builders', as exposed
>> in this wikipedia article:
>>
>> https://en.wikipedia.org/wiki/Set-builder_notation#Parallels_in_programming_languages
>>
>> This is the Python notation:
>>
>> Example 1: {l for l in L}
>> Example 2: {(k, x) for k in K for x in X if P(x)}
>>
>> This is another example in Python:
>>
>> s = {v for v in 'ABCDABCD' if v not in 'CB'}
>>
>> https://en.wikipedia.org/wiki/List_comprehension#Similar_constructs
>>
>> I have been playing with the code below. Nevertheless, I am unsure on
>> how to use the code to define a set.
>>
>> Cheers!
>>
>> <--- Code
>> #!/usr/bin/env perl6
>>
>> my @L = 1 .. 10;
>> my @K = 1 .. 10;
>> my @X = 5 .. 15;
>>
>> say "Example 1:";
>> for @L -> \$l {
>>   print "\$l " if \$l ∈ @L;
>> }
>>
>> say "\nExample 2:";
>> for @K -> \$k { for @X -> \$x {
>>     print "(\$k, \$x), " if (\$k ∈ @K and \$x ∈ @X and \$x < 8);
>> }}
>> <---
>>
>> --
>> (≧∇≦) Mimosinnet (Linux User: #463211)

--
(≧∇≦) Mimosinnet (Linux User: #463211)

(≧∇≦) Ningún Lugar
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
★ Activisme Cultural per a la Transformació Social

(≧∇≦) Fractalitats en Investigació Crítica
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
* Investigació Crítica per a la Transformació Social
* http://psicologiasocial.uab.es/fic
```
 0
mimosinnet
2/11/2019 6:51:36 PM
```Actually I would suggest NOT adding Perl6, because the best way to
create a Set is not to use =E2=80=9Clist comprehension=E2=80=9D, but to jus=
t call
`.Set`

That whole page is about Set Builder Notation, but Perl6 doesn't
actually have such a thing.

You create a Set through a method call, or a subroutine call.

That is true even in the translations I did.

The closest one is where I added a circumfix operator.

my sub circumfix:=C2=AB{ }=C2=BB ( \L ) { L.Set };

I mean this:

Set.new: gather { for L { take \$_ } };

can be simplified to:

Set.new( L );

Or if you're being pedantic:

Set.new( L.Seq );

On Mon, Feb 11, 2019 at 12:51 PM mimosinnet <mimosinnet@gmail.com> wrote:
>
>
> Thanks very much for the answer. I have been playing with your examples i=
n the
> code below (and learned a lot!). Based on your insight, I would suggest t=
hese
> solutions to be added to the wikipedia:
> https://en.wikipedia.org/wiki/Set-builder_notation#Parallels_in_programmi=
ng_languages
>
> Example1: Set.new: gather { for L { take \$_ } };
> Example2: Set.new: cross( K, X.grep: P(x) );
>
> Cheers!
>
> <--- Working code
> my \L =3D 1..10; my \K =3D 1..10; my \X =3D 5..15;
>
> # Example 1
> my \$e1 =3D Set.new: gather { for L { take \$_ } };
>
> # Example 2
> my \$s1 =3D Set.new: gather {
>    for K -> \k {
>        for X -> \x {
>            if x < 8 {
>                take (k,x);
>            }
>        }
>    }
> }
> my \$s2 =3D Set.new: (K X[,] X).grep: -> ( \k, \x ) { x < 8 };
> my \$s3 =3D Set.new: ( -> \x { |(-> \k { (k,x) if x < 8 } for K) } for X )=
;
> my \$s4 =3D Set.new: gather { -> \k { -> \x { take (k,x) if x < 8; } for X=
} for K }
> my \$s5 =3D Set.new: cross( K, X.grep: * < 8 );
>
> say \$e1; say \$s1; say \$s2; say \$s3; say \$s4; say \$s5;
> <---
>
> El Sunday, 10 de February del 2019 a les 12:05, Brad Gilbert va
> escriure:
>
> >In
> >
> >     {l for l in L}
> >
> >The reason it is in `{}` is to create a Set from iterating over `L`.
> >
> >> In Python, the set-builder's braces are replaced with square brackets,=
parentheses, or curly braces, giving list, generator, and set objects, res=
pectively.
> >
> >So in Python:
> >
> >    [ l for l in L ]     gives a list
> >    ( l for l in L )     gives a generator
> >    { l for l in L }     gives a set
> >
> >In Perl6 those would most likely be written as:
> >
> >    L.List   or   L.Array   or   L.list
> >    L.Seq
> >    L.Set
> >
> >---
> >
> >The way to do that is
> >
> >    my \L =3D ((1..10) xx 3).flat.pick(*).list;
> >
> >    set( L ) # A
> >    L.Set # B
> >
> >    my %set is SetHash;
> >    { ++%set{\$_} for L }  # C
> >
> >    # D
> >    do {
> >        # add the {} syntax to create a Set (lexically)
> >        my sub circumfix:=C2=AB{ }=C2=BB ( \L ) { L.Set };
> >
> >        { \$_ for L } # <--
> >    }
> >
> >Something that seems similar to me is `unique`
> >
> >    .say for L.unique;
> >
> >By that I mean, some places where you would use a Set, it makes sense
> >
> >---
> >
> >As for `{(k, x) for k in K for x in X if P(x)}`
> >
> >The easiest one to directly translate appears to be the Scala one
> >
> >    my \K =3D 1..10;
> >    my \X =3D 5..15;
> >
> >    # for (k <- K; x <- X if P(x)) yield (k,x)
> >    Set.new: gather {
> >        for K -> \k {
> >            for X -> \x {
> >                if P(x) {
> >                    take (k,x);
> >                }
> >            }
> >        }
> >    }
> >
> >Other ways:
> >
> >    Set.new: (K X[,] X).grep: -> ( \k, \x ) { P(x) }
> >
> >    Set.new: K X[,] X.grep: &P
> >
> >    Set.new: K X[,] X.grep: &P
> >
> >    Set.new: ( -> ( \k, \x ) { (k,x) if P(x) } for K X[,] X )
> >
> >    Set.new: ( -> \x { |(-> \k { (k,x) if P x } for K) } for X)
> >
> >On Sun, Feb 10, 2019 at 10:26 AM mimosinnet <mimosinnet@gmail.com> wrote=
:
> >>
> >> Hi all,
> >>
> >> I wonder what would be the Perl notation for 'set-builders', as expose=
d
> >> in this wikipedia article:
> >>
> >> https://en.wikipedia.org/wiki/Set-builder_notation#Parallels_in_progra=
mming_languages
> >>
> >> This is the Python notation:
> >>
> >> Example 1: {l for l in L}
> >> Example 2: {(k, x) for k in K for x in X if P(x)}
> >>
> >> This is another example in Python:
> >>
> >> s =3D {v for v in 'ABCDABCD' if v not in 'CB'}
> >>
> >> https://en.wikipedia.org/wiki/List_comprehension#Similar_constructs
> >>
> >> I have been playing with the code below. Nevertheless, I am unsure on
> >> how to use the code to define a set.
> >>
> >> Cheers!
> >>
> >> <--- Code
> >> #!/usr/bin/env perl6
> >>
> >> my @L =3D 1 .. 10;
> >> my @K =3D 1 .. 10;
> >> my @X =3D 5 .. 15;
> >>
> >> say "Example 1:";
> >> for @L -> \$l {
> >>   print "\$l " if \$l =E2=88=88 @L;
> >> }
> >>
> >> say "\nExample 2:";
> >> for @K -> \$k { for @X -> \$x {
> >>     print "(\$k, \$x), " if (\$k =E2=88=88 @K and \$x =E2=88=88 @X and \$x =
< 8);
> >> }}
> >> <---
> >>
> >> --
> >> (=E2=89=A7=E2=88=87=E2=89=A6) Mimosinnet (Linux User: #463211)
>
> --
> (=E2=89=A7=E2=88=87=E2=89=A6) Mimosinnet (Linux User: #463211)
>
> (=E2=89=A7=E2=88=87=E2=89=A6) Ning=C3=BAn Lugar
> =E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=
=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=
=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=
=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=
=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=
=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=
=AC=E2=96=AC=E2=96=AC
> =E2=98=85 Activisme Cultural per a la Transformaci=C3=B3 Social
>
> (=E2=89=A7=E2=88=87=E2=89=A6) Fractalitats en Investigaci=C3=B3 Cr=C3=ADt=
ica
> =E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=
=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=
=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=
=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=
=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=
=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=AC=E2=96=
=AC=E2=96=AC=E2=96=AC
> * Investigaci=C3=B3 Cr=C3=ADtica per a la Transformaci=C3=B3 Social
> * http://psicologiasocial.uab.es/fic
>
```
 0
b2gills
2/11/2019 11:04:14 PM
```RWwgTW9uZGF5LCAxMSBkZSBGZWJydWFyeSBkZWwgMjAxOSBhIGxlcyAxNzowNCwgQnJhZCBHaWxi
ZXJ0IHZhIGVzY3JpdXJlOg0KDQo+QWN0dWFsbHkgSSB3b3VsZCBzdWdnZXN0IE5PVCBhZGRpbmcg
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DQo+PiAgICAgICAgICAgICAgICB0YWtlIChrLHgpOw0KPj4gICAgICAgICAgICB9DQo+PiAgICAg
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```
 0
Joan
2/13/2019 1:19:37 AM
```Hello folks. Did I understand correctly that this thread is about list
comprehension syntax in Perl 6? :-)
I don't if it was mentioned, but I think this syntax just simply works

> say (\$_~\$_ if \$_ %% 2 for ^10).Set
set(00 22 44 66 88)

> say (\$_.item if \$_[0] eq 'a' or \$_[1] == 2 for <a b c> X ^3).Set
set((a 0) (a 1) (a 2) (b 2) (c 2))

(Just drop the .Set from the end if it's not needed)
```
 0
lucasbuchala
2/22/2019 5:41:14 PM
```Yes, +1 and we have this documented on the py-to-perl6 nutshell page:

https://docs.perl6.org/language/py-nutshell#List_comprehensions

On Friday, February 22, Lucas Buchala wrote:
> Hello folks. Did I understand correctly that this thread is about list
> comprehension syntax in Perl 6? :-)
> I don't if it was mentioned, but I think this syntax just simply works